<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        let num = +prompt('请输入一个数字');
        num < 10?alert(`0`+num):alert(num)
    </script>
</body>
</html>